A couple of drops in a bottle or tank will work fine. If you stick to flavours you're confident will go well together, you're unlikely to make a bad juice any worse.
When I'm testing a new blend or new concentrates, I will often blend directly into a tank, add some unflavoured base I mix up in advance (remembering to factor in the percentage of flavour to be added later, to keep nic, PG and VG at the desired ratios) then give the whole thing a REALLY good shake. It won't be steeped, so not the perfect, rounded flavours it will develop over time, but gives a good indication of whether I'm on the right track (and they do steep in the tank if I'm flitting across flavours and it takes a while to get through it).
Once you do have a good idea of preferred balance, it's always best to then go with a bottle so it can be steeping (even if you can't wait to vape a bit of it, the rest will be maturing).
For those who like menthol, that would probably blend well with a lot of fruit flavours (you can buy that in cherry, raspberry and others).
Coconut goes with lots of fruits and with rum, cola etc
Creams, caramels, rum, coffee, cinnamon, nutmeg, ginger, chocolates all blend well with each other
Of course virtually all fruits will compliment each other
I would've thought most tobaccos would blend well with practically any other flavour
Just remember that you will slightly reduce the nic content and increase the PG (assuming your concentrates are PG based) with every addition to an existing juice. Probably not enough to make a significant difference unless you get a bit carried away.
