I guess I'd ask the question Why are the other 3 APs on a different VLAN? It's unlikely to be because of scaling concerns, I'd have thought. With so few APs, it's unlikely to be because you have thousands of clients, all on the same broadcast domain...
If those three APs are geographically separate, they'd have to have overlapping WiFi cells between the two locations, to take advantage of any roaming anyway.
I like the Meraki 'keep it simple' approach, extended to config here: only L3 roam if you have to. Assign all clients in one SSID into the same VLAN, on a site (any given Network) if you can.
Depends... (I hate answers like that). I’ll elaborate. Can users transition seamlessly between floors, maintaining their wireless connection? In my experience generally they can’t, they have to either use a stairwell or a lift where there is no wireless connectivity, and so the device will disassociate and have to re-associate anyway, so no need for roaming. Occasionally you may get an office where it is possible for clients to move between floors, and then I may just have a single subnet for those floors, but that’s the exception rather than the rule - generally I find it’s one subnet per floor, no need for roaming between them. If they can move between floors without disconnecting, they do, and they have an application that requires seemless roaming (e.g. VoIP), then at six floors I’d probably consider Layer 3 roaming rather than a large subnet. But, again, that depends on how many devices I’d expect on the subnet.
From a subnet sizing point of view remember you may have a subnet for corporate devices, one for BYOD, one for guests, so I’d say a /23 is the largest you should ever need - beyond this the broadcast domain gets too big in my opinion.