Hi, I'm a Page Plus customer, dinc user, and a recovering ROM flashing addict.
First off, what are you 'flashing' to switch to Page Plus? You just need to dial *22890 to program it, after the ESN is activated on Page Plus.
And really, you can flash whatever ROM you want. If it works on Verizon, it'll work on Page Plus. Of course the usual rules about being rooted, etc. apply... I've used MIUI gb, MIUI ics, AOKP ics, Evervolv ics, and am currently running Ice Cream Senseless because I got tired of the camera issues on the ics roms.
The only thing you need to remember is that you need to use Page Plus's codes when doing OTA programming and PRL updates. It's *22890 to program/activate the phone on Page Plus, and *22891 to update the PRL.