Festo
I've done exactly what you're trying to do with Festo blocks, and I don't remember any major difficulty.
There should be some DIP switches (incredibly small ones, as I recall) on the Festo board. You'll need to look in the Festo manual to find out what the settings are to set these to the rack size and starting block that you want.
From the PLC's perspective, the Festo will appear on the RI/O link as any other rack of PLC I/O would.
The smallest "rack" size you will be able to make the Festo transmitter 'appear' as is 1/4 rack = 2 x 16-bit words = 32 Festo blocks. If you have more than that under one controller, then you'll want to up the size of the block, but more than likely, you don't.
If you have multiple block controllers (I don't remember what Festo calls their ASB/Remote I/O adapter), then you will give them a different address than eachother. You might set one up as Rack 1, 1/4 rack, starting slot 0 (which will come into the PLC as I:10/0 through I:11/17 (octal addressing)), and the second as Rack 1, 1/4 rack, starting slot 2 (I:12/0-I:13/17). This assumes that the PLC chassis is an 8-slot rack, not a 16 (a 16 slot rack, with single-slot, non-complimentary I/O, would occupy all of racks 0 and 1 in the I/O image table, so your first available Festo rack would be 'Rack 2')
As for setting up the remote I/O in the RSView, you just double-click on I/O configuration, right click on the I/O Channel column, and select "Add Chassis". Select a "Custom I/O device" with "Generic RIO Chssis", enter the size of the racks, with the same configuration settings as you set the Festo DIP switches, and you should be good.
I had thought that there was an Auto-Detect for Remote I/O, but you have to be online with the 5/15 to see that option, and I don't happen to have one on hand (5/25, 5/11, sure. 5/20, 5/40E, no problem. But no 5/15. Sorry).
I don't remember about the S+ and S-. There should be 3 wires - 1,2,Sheild - perhaps those are them, with the shield somewhere else. Again, your Festo manual should say.
There's a good chance that when you connect your remote I/O up, you'll have the wires backwards. If at first you don't succeed, swap the blue and clear on one end.
Getting RIO up and talking is almost always a PITA. Start slow, with just one block as the OMLY RIO device. Then do another as the only one. THEN the two together. This will save you much heart/headache when troubleshooting (is it the cable or the switch settings or the configuration or ....) by starting off knowing that you don't have address conflicts, or have to redo a lot of wiring, or....
AS for terminating resistors, if you use a fairly short (10') cable, while you try to get everything talking, you probably don't need them.
DH+ simultaneous with RIO? Yes, absolutely. You should have two dedicated channels - one labelled "Peer to Peer Comm" (AKA DH+) and the other "REM I/O") (these are off of my 5/25 - same family).
A few vague memories (which, being older than 24 hours are suspect): We had some problems with the connections between the Festo ASB board and the control block backplane. Loose connections or some such thing. Also at least one backplane went bad (did a bit of cannibalizing to make things work). The connections seemed a bit flimsy. But the blocks themselves worked well.
Good luck. As always, post any further questions. (And that's POST , not Private Message).