Echo Kinnem's response that there is probably nothing legally prohibiting participating in AROTC (College Program) while pursuing Marine PLC, and I also have no idea what the OSO would think but I will share that were I the OSO, I'd find it....odd. Do you simply want to serve (which is noble) or do you want to be a Marine and leader of Marines? The challenges to earning a commission are many and I would recommend picking a path and committing to that rather than what I would see as "hedging my bets".
I'll also share that it is common for NROTC MO (College Program) to begin discussing PLC with their OSO while still participating in the NROTC MO program. You'll want to have a good relationship with the NROTC Marine Officer Instructor (MOI) AND the OSO. It's not uncommon for College Programmers to compete for the 4-year National Scholarship as a college freshman...not be selected...pursue a side-load as a sophomore....not be selected and then leave NROTC MO to accept a PLC contract.
Reminder that PLC isn't a given...it's competitive as well.
I don't know all the details and know that Senior Military Colleges have special rules, but from what I have read, the language is rather Army specific. In other words, I don't know if the guarantee of a commission applies equally.
I don't have data to support this, but my sense is that the number of side-loads has decreased in the last few years. I suspect (but again, no data) that that is partly due to the changes in the 4-year National program. Now, you basically have 2 chances (4 looks) at the 4-year scholarship (i.e. early board during Senior year of High School and again for the late board and then you can apply again during your Freshman year of college for two more looks). The MOI has significant input into that Freshman year application....
Last, on the PLC/NROTC interaction. My personal experience (I was a product of MECEP - an enlisted to officer education/commissioning program) was that it is true that the PLC folks don't do the NROTC stuff (drill, classes, social events) but our PLC candidates came to Marine PT every morning with the NROTC MO candidates and went to our field exercises as well. The OSO and MOI had a close relationship and it was clear the PLC candidates were expected to take full advantage of the Marine oriented events...
Twenty-something years later and my son is experiencing the same thing at a different university. The PLC candidates are encouraged to come to Marine PT and they go on every Field exercise with him and his fellow NROTC MO candidates, at least until they've completed OCS (or PLC juniors/senior or PLC combined/OCC).