Alright, cool I thought Yoropiko fit slimmer though? At least that's what I heard.

Some Yoropiko are slimmer in the leg and seat, eg StarWars, but the waist is unaffected.

I wear size 36 in true religions and their a bit saggy on me so what size should I get for rmcs and yoropiko? A 38 or 36?

Probably not a bad idea, Ive got 2 pairs of T.R. and to me they seem smaller than their stated size, probably more to do with the cut though as both mine are the Billy and Logan styles. I think you can def. stick to 36 for Yoropiko as I find they tend to be slightly looser than the same size in RMC in my experience, although there are slimmer styles of Yoropiko where this isnt the case just to confuse things.
However of you want a good way to cheat to find out your proper size pop down an Evisu dealer and try a pair of their Raw selvedge jeans, the cut and fit is pretty much identical to lot 1002 RMC so you can find out what fits best before commiting to a purchase, especially if you are buying from say Togged online.
You might find you are a 34 in RMC after trying it out, but of course the size you buy will also depend on how you wear them too. If you have them hanging off your waist loose style then of course you will need a bigger size than if you wear them normally like I do for example. If you like to sag, then the opposite will be true and you will need a size up.
Incidently, when you measure your waist what size is it, if it is actually say a 35 then stick with 36 for RMC and possibly consider a 34 in Yoropiko again depending on the style.