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As long as they don't use the same IP addresses, that shouldn't cause problems. It's worth avoiding if you can, simply because that's an easy way to guarantee it won't interfere rather than shouldn't. Ounce of prevention and all that.

As for sync capacity, that really depends on the connections per second which the firewall handles. 1g is generally plenty of capacity for the 1U boxes. If you want fault tolerance, bond two 1g interfaces together. I wouldn't bother with 10g for sync unless you're doing some ridiculous stuff like syncing all connections on a firewall in front of a DNS server.

Your comment about the VLAN for sync makes me pretty sure you know this already, but you should run sync through a switch (or a pair of switches for a bonded pair of sync interfaces).