Does the IOTV require soft armor inserts when used with SAPI plates? Also, will a large plate work in an extra large carrier?

I wouldn't use an IOTV with just hard plates. You "can", but it will sag and be generally awkward. If anything I would use the IOTV with just soft armor and no hard plates. If you only have plates I would just get a plate carrier.

As far as size of plates I believe the iotv plate pocket ranges up and down one size. So a large should be able to use medium, large, extra large.

There is a lot of hate for the IOTV due to the weight...but personally I think it is a great vest if you want maximum soft/hard protection.

I have 4 ceramic plates. One large, and one extra large. I also have the side plates. What carrier would you suggest? I am really only thinking of home defense, not as any thing regularly worn. But are the ceramic plates safe without soft armor?

Depends what type of plate it is. If it is rated as a stand-alone it is fine to wear without soft armor, but if it is  ICW (in conjunction with) it will require 3A soft armor to be totally safe. The reason being is that when a round strikes the ceramic it can send shards and pieces of the plate into the wearer and back face deformation can damage the body as well. You can find all this info on the plate itself.

There are TONS of plate carriers out there for you to choose. My thoughts on use of armor for home defense is that you don't need velcro and elastic slowing you down. Basically throw it over your head and rock and roll. In that regards you can pretty much get a LBT, mayflower, TAG, or any other PC. All it has to do is hold your plates, a mag or two, flashlight, and an aid-kit.

As a catch all vest the IOTV is pretty damn good. It has huge coverage of soft armor and has plate pockets. I like the IOTV as a catch all because if it is feeling too heavy I can drop the collars and hard plates and still have a comfy soft armor vest.

if you are set on getting a IOTV, step up and get the MTV or Spartan II.  They both take interceptor soft armor and extras but are a cummerbund front opening.  still a catch all but much improved mobility and durability.  Should be pretty cheap since the military is pretty much done with all forms of frag vests it seems.

edit- IOTV's are a lot like the MTV, I was thinking the old interceptor.

OP I hope you don't mind. I was given a old interceptor vest. I want to use the soft inserts and plates(future buy when I can) in a modern cummerbund style carrier. I have been looking for a Spartan II or similar for this reason. I want to use the free soft inserts and when I can afford it get plates. I am only using this rig for active shooter, method raids, etc. Vollie medic, no budget, no tactical medic, zombies, SHTF,etc. Several times running calls that where not what dispatch said, I thought for sure I was in trouble if I followed the no medic/FF can be armed. County LEO is thin, budget cuts, etc. Doing what we can with what we have.

I know you should be okay with just soft and no plates OP.

If anyone has a carrier that will use interceptor panels, has cummerbund and side plate holders please let me know where to look.

Thanks