I forgot to say, you can refill these about 5 times (this means about 5 x their 20 drops capacity - so about 10 x 10 drops maybe) before they start to die off due to getting crudded up. At that stage you can wash them out or throw them.
Anything will fit, if you use an adapter. The V4L series is the KR808 model, so to fit 510 series stuff you'd need a KR808-to-510 adapter from Madvapes, Good Prophets etc. However the batteries are not big enough to drive some LR stuff satisfactorily. In other words, don't attach anything with a resistance lower than 2 ohms or it may not work well, and it may shorten the life of the battery. In practice this means you can attach standard cartos from another series, with the adapter. Or an atty & cart, or even better an atty & driptip as that is the best system for testing flavors.
There wouldn't be much point in attaching a 510 Boge carto as that is basically what the V4L cartos are, with a KR8 thread (V4L cartos are from Boge, liquid is from Dekang). Boge-Dekang is the same firm, one side does hardware, the other side does liquid.
By all means get an adapter so you can attach a 510 atty and driptip, as that is kind of a benchmark set-up for testing flavors. After that you might be better off going for a bigger battery model, at least for use at home even if you don't as yet want to use it out & about. Others will maybe chime in here with what works well with your existing KR8 cartos, there are many options, changing all the time.