Looking to use thermal for varmint's. Does anyone make a optic that can be mounted in front of my scope? What manufactures are recommended and who should be avoided? I know nothing about thermal optics, any help is appreciated.

Super Yoter C or one of the clip-on hogsters. They can clip on rails or clamp onto scope objectives.  Downside is the mounts are like $200/ea, so if you have a bunch of different scope objective sizes you could be looking at an extra $800 in adapters alone.  Also they are not true clip-ons; they get digitally collimated when zeroing and the offset for particular rifle is saved in the settings. So if you’re rail mounting, forearm flex will push your zero.  Objective mounting is probably ideal.

A thermal clip on with true integral collimating prism (doesn’t matter what optic you put in front of) is probably going to be well into the 5-figures.

I went round and round between the Thermion 2 XP50, Super Yoter R and Super Yoter C... I thought the clip on was the most attractive and seamless solution but I would have needed 4 different mounts for the guns I wanted to use it on. It would have been a couple hundred more than the XP50.  I really wanted to want the XP50, but it’s battery situation was too much to stomach.

I ended up settling on the Super Yoter R and I’ll swap the day optics out for the thermal when necessary.  Most quality optic mounts will retain zero great. Proetty much all the thermals allow for programmed zeros of several different rifles and the Yoter R comes with a LaRue mount.

Another vote for the yoter.  That said, at this point I would wait for the model update this year that will now include an integrated range finder.  Super helpful if you hunt in places with visibility over a few hundred yards or with subsonic loads.

Just grabbed a new Sig unit. Should arrive this week.

Please clarify what you mean by varmints? Skunks, raccoons, etc in your backyard is very different than calling coyotes in open country, etc. The following questions are critical to help with recommendations:

1) What is the intended usage?
2) What type of terrain will it be used in?
3) What would you consider your minimum shot distance, average, and maximum range needed?
4) What is your budget?