1) The concentration will affect both fixing times and capacity
2) Yes, with an acidic rapid fixer there is such a thing as overfixing. Proper fixing involves the removal of silver halides which were not developed into metallic silver. If you keep going far beyond that point, the fixer will gradually start to bleach the metallic (image) silver also.
To add to Michael's post (quite) a bit.
Using Rapid Fix 1+9 with film works fine, but does reduce capacity/volume and increase the fixing times. Some years ago I experimented with using dilute fixer and communicated with the Ilford tech lab and others at the time. Long story short: there is a range of dilutions where fixer works well. Too dilute and too concentrated and the fixing will be inadequate. 1+9 is about as dilute as I found to work well and still be practical. Higher dilutions (as I understand what was communicated to me) risk interrupting the fairly complex chain of reactions that change the silver halides into soluble compounds by not supplying enough ions at the right place at the right time. Ilford Rapid Fix at 1+4 is the "sweet spot" for film and this is the dilution I use most. However, when I have just a couple of sheets I need to develop, I can still get a good fix by using 1+9 and save a little fixer in the process, since I use my fixer one-batch always. (Too concentrated fixer will not fix well at all; it needs the water.)
Both capacity and fixing times for more-dilute-than-usual fixer can and should be determined by a standard clip test. Just so you don't have to look it up, here's the best practice for doing the clip test: The entire test should be done in good illumination. Place a drop of fixer on a small piece of film and wait 30 seconds. Then immerse the entire piece in fixer, start your timer and agitate. Keep an eye on the place where the drop was; it'll clear first, with the rest of the film 30 seconds behind. When the film clears to the point where the place you put the drop and its surrounding area are indistinguishable, note the time. This is the clearing time. It is important to benchmark the clearing time for your various films in fresh fixer (and for each dilution you plan to use). Do the clip test and write them down
Do a clip test before each batch of film. Use a
minimum of 2x +10% of the clearing time as your fixing time (this changes as the fixer loses activity with use, so keep an eye on this). When the clearing time of the used fixer reaches 2x that of fresh fixer, it is exhausted and needs to be replaced. Many use two-bath fixing for prints; I advocate it for film as well if you have a large volume. Search for two-bath fixing if you're interested.
Caveat: if you fixed a lot of film in weak fixer, you could have easily exceeded its capacity. If you've still got the fixer, do a clip test and compare it to fresh fix. If you have exhausted fixer, you should likely re-fix the negatives in question (the fraction of the batch you feel you've underfixed plus a buffer).
A note on fixing time for film: film is coated on a waterproof base that does not absorb fixer. Extending fixing time past the minimum determined by the clip test does no harm up to a certain extent, eve with acidic rapid fixers. Ten minutes is about the limit here. Much more than that, and the image will start to bleach a bit. That said, a tiny bit of image bleaching is often inconsequential with negatives. Still, it's better to stay under the 10-minute mark. Many recommend a standard five or six-minute fixing time for film in rapid fix, and use the clip test only to determine when the fix needs to be discarded. I am one of those
So, even if my film clears in 45 seconds, I still fix for 5+ minutes. This not only ensures adequate fixing with a generous safety factor, it also helps get rid of the pink tinge that a lot of modern (esp. Kodak) films have.
Just an aside about manufacturers' recommendations, etc. Yes, they are best and safest practice. However, there are other ways and one does not have to slavishly follow directions if one knows what one is doing. (If you don't, then by all means, "RTFM," but if you do, then it's up to you to make the best choice for your situation). I find calling people to task for not following directions, especially when there was little choice but to do otherwise not only rude, but closed-minded. The OP's question indicated an unusual situation, not necessarily sloppiness.
Best,
Doremus