everything within a VLAN MUST have a matching MTU size. Your routers MUST also be capable of sending ICMP fragmentation needed errors (most do, automatically) so that hosts can discover path MTU and successfully exchange data to hosts that are not running jumbo frames (such as internet destinations).
This doesn't just go for jumbo frames, it applies to everything - someone on cable internet with an MTU of 1500 can talk to someone else on PPPoE ADSL with a 1492 byte MTU because pMTUd is what enables the cable internet user's host to know that they can't receive frames > 1492 bytes.