Hi Txema, Peder,
It's usually best to charge the batteries with a same brand charger to avoid issues like warranty problems if something goes wrong. Globalmediapro's Li-Ion batteries use simple charging and will not properly work with intelligent charger like Anton Bauer's. I don't know the software in GMP's chargers but IDX VL-4S is well protected and charges in various stages. SWIT chargers are not visually too much different from GMP's so they might have pretty similar electronics as well so my guess is that they might not be as advanced as IDX's.
I've been able to charge any V-mount battery with IDX chargers (except LE-UB-8/-12 of course). IDX chargers have detected some problems like imbalanced cells in a battery when the original charger have not.
When you mix different brands you should keep eye also on the battery contacts because there are so many different versions & generations of those tiny banana plugs and sockets that they most likely don't match perfectly - but this is the case even with IDX batteries and IDX chargers - so if you see any marks of "welding", burnt or discolored metal (due heat) you propably should not use that combination any more.
To make it general, are the output amps of the charger what I have to Watch out for?
- Yes. These v-mount battery chargers usually charge batteries with 2-3A and I wouldn't charge with any higher Amps. (And you must remember the max output current rating of your battery doesn't mean you can charge it with same current.)
I'd say IDX is safe if not actually the best option, Swit's charging current should be checked from the specs before use.
I'm not giving you the definitive answer but hope this will help you to decide
If still in doubt you can send an email to sales[at]globalmediapro.com - they are really nice and will quickly answer your questions.