Internally and externally you can use pretty much anything that is made for TM. The only catch to Classic Army M4/M16 models is externally when it comes to mounting RIS/RAS units. The upper receiver on the CA's are a bit higher than Tokyo Marui, and other brands.
You can certainly mount a railed front end onto a CA, but keep in mind that the top rails on the front end and the top of the receiver won't be at the same level. The RIS/RAS top rails will be slightly lower. This also means that most brands of CASV rail system are incompatible with the Classic Army receivers.
This refers to CA's metal body models. However, I am going on the assumption that the plastic body Sportline versions are made to similar dimensions as their metal counterparts.