Hi Bevan,
I agree with Sam that if you were to do shallow scuba dives, it's a bit like doing deco. I think that scuba after freediving should be fairly safe, unless you are really pushing a lot of deep freedives in a short space of time. (Or using a sled etc)
I've done quite a bit of freediving before and after scuba (my bad). Having said that I never did any freediving after deep scuba, or one that would put you in a high nitrogen group. It was mostly not real deep freediving either, not much over 20m. What I did do if I knew I would be freediving after, is an extra long safety(deco) stop. So instead of 3-5 minutes at 5m, I would do 15 or so.
I guess if you could calculate the nitrogen loading with freediving, and had some sort of table, then I see no reason you couldn't mix both. So you could treat the freediving the same as scuba, work out surface intervals, safety or deco stops etc. Of course no one has done this yet so the risk is if you get it wrong. :waterwork
If you want to add an extra safety margin - after your freediving you could put on your scuba gear and do a 15 or 20 minute safety stop at 5m or so?
(Work even better if you had nitrox). Then after another 2hours surface interval start your scuba. Add in some extra deco on that as well.
P.S. I don't recomend freediving after scuba, I'm just saying I've done it. :duh
Cheers,
Wal