The IF doesn't have anything to do with the channel spacing at all; many older handheld transceivers have IFs of 455KHz, and that has no impact on the channel spacing.
In simplified terms, minimum channel spacing is determined by the modulation bandwidth. If you've got FDMA (i.e. normal AM, SSB or FM modulation) then the channels are slightly more than the modulation bandwidth apart. So if you have a 3KHz modulation bandwidth, the channel center frequencies would be 5KHz apart.
If your modulation scheme is based on spread-spectrum techniques, then basically everything changes, and everyone shares the same frequency band with separation of signals provided by receive correlation.
Now, to your other question of how the channels will be allocated? That's only barely a technical issue. The regulators in the particular country you intend to operate that transmitter in have regulations and band plans that say what can be transmitted where. They usually specify bandwidths and center frequencies for FDMA, add slot timing to TDMA, and specific spreading codes or classes of spreading codes for CDMA.
So, in the US, the FCC controls channel spacing for duplexing, and I don't remember (and don't have at hand) the spacing in that frequency band in the US. -- Steve Watt KD6GGD PP-ASEL-IA ICBM: 121W 56' 57.8" / 37N 20' 14.9" Internet: steve @ Watt.COM Whois: SW32 Free time? There's no such thing. It just comes in varying prices...