Originally posted by micromacro Stupid question: what does that mean "go manual" in the case of flash?
It means there is no automatic exposure support, i.e., the flash power will be set by yourself, the camera does not assist (which is a good thing in most situations!).
Originally posted by micromacro . I have old promatic TCL2450 flash, untested, one battery contact is a bit corroded, but easy to remove. In case it's working flash, can I use it as a second flash and with cactus transceiver?
I couldn't find any information about that flash. As long as it does not have negative polarity, you can fire it with the V5 and V6. If it has a TTL mode, you may be able to remote control its power using V6 triggers.
Originally posted by micromacro And last question Should I buy light meter to understand better how flash-ambient light work together?
I don't think you need a light meter. They were most important in film days, but nowadays you can simply check your light levels by doing a couple of test shots. There are still a few applications for lightmeters, but I'd say they are rather specific and if you don't know about them, you don't need a lightmeter.
Originally posted by micromacro Do I understand correctly that YN560 IV will work on any camera system (so, I use the same flash for Pentax/Canon), and also saves money for transceiver?
Yes, any camera system can fire the YN560-IV.
However, there is only one trigger that can remote control its power and I don't think the YN560-TX is as good as the Cactus V6. I wrote a
comparison between the YN560-III and the Cactus RF60 and while that was before the YN560-TX was revealed, it may still give you some hints, in particular, if you read up about the YN560-TX elsewhere. Just be mindful that some sites like TruthHavoc (its real name is "FlashHavoc") try hard to appear impartial, but the information is presented in biased ways, exaggerating in one case, leaving out information in another whenever convenient, censoring comments, etc. Treat it as one source of information as opposed to gospel, and you'll be fine.