Foams will definitely be the way to go given the mix of tarmac, carpet and wood, however...
I think unless foams are available trued & glued for a decent price it would be worth considering a 'rubber tyres only' rule. Requiring people to prepare rims and glue/true foam tyres will be a good way to kill this racing, as it's a messy job and requires tyre lathes, adaptors etc.
Here are some foamy threads:
http://www.oople.com/forums/showthread.php?t=32648
http://www.oople.com/forums/showthread.php?t=39366