The answer is "It depends..."
If you configure both interfaces on the 2901 as trunks and you configure the
trunks to forward all of the applicable VLANs, and you configure the interface
on the 1900 as a Trunk, then it will work just fine. If you get any thing
wrong, it will break.
As to different ISL groups, I'm not sure what you mean. There are no ISL groups.
ISL is ISL. It encapsulates a VLAN number onto each packet going onto the wire,
and decapsulates the packet into the VLAN on the receiving switch. As such,
you could do the following (which is what I think you're aiming for):
Cisco Router
Subinterfaces F0/0.2-500 == VLANs 2-500
Subinterfaces F0/0.501-999 == VLANs 501-999
2901
Trunk Port 1/1 == VLANS 1-1000 and connects to Cisco Router
Trunk Port 1/2 == VLANS 1,501-999 and connects to 1900
1900
Trunk Port 1/1 == VLANS 1,501-999 and connects to 2901
Then you could put users on VLANs 2-500 on the 2901 and users on VLANs 501-999
on the 1900, and be just fine. Additionally, if you wanted to add a VLAN to
the trunk between the 2901 and the 1900, say VLAN 250 (so that the new
list would look like 1,250,501-999), you could even put some people in
VLAN 250 on each switch and still have them talking to F0/0.250 on the
router.
Hope that helps.
Owen