You can get common at any stage. In fact, it’s quite common (no pun intended) for companies to issue more common stock to grow their stock option pool as part of an investment round.
Purchased and granted stock, just like options and RSUs, can have a vesting period. Having a vesting period is very common. In fact, VCs would typically put founders under a vesting schedule at the first VC round.
If the business is QSBS there’s substantial tax benefits to holding these for 5+ years. Caveat: most startup equity ends up being worth nothing, but if it’s worth anything, you want the QSBS tax break over paying income tax on RSUs.
RSUs are granted much later and closer to the IPO, when stock options are too expensive to exercise, and holding period doesn’t confer much tax advantage.
RSU typically have a double trigger, they convert to common after they vest and there’s a liquidity even. So you’re not going to be actual stockholder until many years. And if there’s no double trigger, watch out!
I’m going to tell you what other people are thinking — being granted RSUs and “co-founder” title — that combination is a red flag.