So, tried it out, works, slight space , barely a playing card width of space between rear of upper and buffer, cycled by hand, overall appearance is a bit goofy when looked at closely, rear takedown pin took a gentle tap to secure the upper, but wasn’t at all difficult/ or forced . Only issue I’ve seen thus far is bolt catch on Fulton Armory lower isn’t playing nice with the m5 upper.( seems tighter than it should be if that makes sense) M5Enchanced to be more specific, with AREO BCG. Does the bolt catch signify an unseen/internal issue? I can depress the catch to lock BCG back, but trying to release it via the catch is tricky. Locks back on empty mag, and is setup with A2 style stock/rifle buffer as well. Tinkering will continue, just wondered if anyone else has tried this arrangement and how’d it work out if so, thank you, edited to add the bolt catch thing doesn’t really make much of a difference to me IF it’s functional 100 percent really, this will probably turn into a bench/target plinking set up, so manually working the CH on a fresh mag isn’t that big a deal for me.  More concerned that something internally is going on and I’m missing it or overlooking it, main concern as always is safe operation

Thank you for posting this and keep on updating. The only way to know which combinations work is from people like you trying them out and letting us know. When you close the upper, does the tail of the carrier push the buffer off the retaining pin? What's the distance between the bolt face and the bolt catch at full pull back of the charging handle? How's the charging handle clearance from the top of the RE hub?

I’ll post pics of them fitted together shortly, I’m still having trouble posting pics , got some up the other day on a trade thread, so I imagine I can do the same here, and upper does not interfere with buffer retainer at all. There’s an adequate amount of clearance between BCG and ejection port when empty mag has locked bolt back, as well as when I manually engage the bolt catch. Areo  Ambi charging handle has plenty of clearance over the rear top of lower as well

https://imgur.com/a/R7GQbfd
A few quick pics, and I misread the question as to BCG/buffer contact, yes when the upper is seated on lower the buffer is slightly compressed, hard to see in a pic, but you can feel the spring and buffer retract towards rear, for whatever reason I read question as , does the BCG  press down the buffer pin.  Pics show bolt open  on bolt catch, but it can go back in tube further than I was able to get a pic of. Also showed alignment of rear take down pin, both in and out best I could, lug on upper lines up pretty good and has already started to get easier to assemble and take apart. Bolt catch felt gritty even with out upper on, definitely has some wiggle back and forth, popped it off and ran some 500 grit over the contact points , barely enough to remove the finish, and gave it and the catch groove a good cleaning, works better now, but not great, may just try to replace it. Other than that, it’s a pretty decent fit up, and looking forward to taking it out to function test soon.

One issue with mixed receivers is bore alignment. The space between the CH and the top of the RE hub can be a indicator. If this was my build, the gap in your picture would warrant further checking. One way to check alignment is to see how concentric the BCG is to the RE hub, which has to be done before the RE is installed. Otherwise, keep an eye out for unusual wear on the inside of the RE and sometimes the BCG.
The gap between the rear of the upper and the RE hub can effect how much the carrier pushes the buffer back so the buffer doesn't strike the retaining pin. It's probably enough. but watch for buffer face damage.
As far as the distance between the bolt face and the bolt catch, too small of a gap may not allow enough time for the catch to pop up and stop the bolt after the last shot. Too much of a gap and the momentum of BCG/buffer may break the catch. I measure that by installing an empty mag and pulling back the CH to engage the catch. While maintaining slight rearward pull on the CH, mark the top of the CH at the tang where it exits the upper. Then pull the CH fully rearward and make another mark. Measure between the two marks, I like 0.150" - 0.200" but some go up to 0.250".
These are just my opinions and I don't want sound too negative. Good luck!

Thank you, all very good info! I’ll measure the charging handle as you suggested tomorrow and confirm the travel between the two marks, that’s a great idea. Didn’t even cross my mind that there was a “too much/too little” possibility as far as that went. The 308s are a new thing for me, i built another with an m5 lower, and a standard m5 upper, and also chanced upon a great deal on the complete m5E upper/ handguard. Rather than swapping back and forth and reconfiguring my set up I figured I’d find a permanent home for the enhanced upper, and took a chance with the Fulton as I was considering  something with a fixed stock anyway. If it doesn’t seem to work I’ll just keep an eye out for an m5 and put the Fulton to the side for the time being. I’ll update my findings as I go, thanks again

CH measurement was just on the high side of .250, took it out this morning, functioned great. Only 20 7.62X51, 147 grain, and 20 .308, also 147 grain. Strong 3oclock ejection pattern every time, and bolt held open on each 10 round mag without any issues. So I’m gonna say that’s a good start. And no buffer ding or damage, no strange wear anywhere on BCG or inside of receiver either. Only 40 rounds, but still promising

Sounds good. If you wanted to tighten up the distance between the bolt face and the catch, you can drop a couple of quarters down the RE before you put in the spring and buffer. I use 15/16" Delrin rod stock cut to length, but the quarter trick is tried and true.

Nice, another good tip:) I was actually considering punching a few circles out of some thin sheet metal stock with a hole saw for shims, but the quarter sounds way easier??