ZD3000 (25 AP license) and 2 AP's - seamless raoming doesnt work, clients gets disconnected while moving from one to another.
1st ap - 7962
2nd AP - r300
When client moving from one ap to another client doesnt disconnect from the weak signal of the first ap and consequently doesnt connect to a much stronger signal of teh second ap.
2. Do we need a similiar versions of firmware on all APs?
If you have the "Fast BSS Transition" enabled on the SSID from the GUI, then you need to set the "Roam-Factor" where the AP will decide to disconnect the client once the client SNR reached to certain threshold. This way the client can reconnect to the nearest AP and will not stick to the connected AP even tough the signal is weak.
Check the below article to set the roam factor.
On your second point the ZD image remains same for all connected APs, so answer is Yes. For your above two pointed AP models 184.108.40.206.14 is the maximum code on ZD3k which will support both 7962s and R300.
Adding to Sanjay's comment on the WLAN can you check if 802.11k is enabled ?