There is a big dillema even among more experienced players whether maximizing profit lies in focusing on 1 format or in mixing 2 or more formats at the time. Answer for this question is not a simple one but I'll try to give you some hints on what it seems can work out for the majority of population.
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Lets first start with the advice for completely new players. When you are newbie in poker the idea for you should be exploring and experimenting as much as you can in order to get the idea what seems most attractive and most comfortable in the world of poker. Another thing that i always stress out, do not play with money you cannot afford to lose and ideal would be not to care about money at all when beginning. Serious poker can't be explained in short term achievements but i do understand there are some of you in it for just some fun gamble and spending your free time in amusing way and that is completely fine too but then you have to sort this in your head and know it is all just good fun win or lose. So advice for all newcomers mix as much as you can when you eventually come to the point you realize you make more profit/ feeling most comfortable/enjoying one format more than others so you are ready for your next step.
Alright, now you already see poker as something more serious in your life, you already have some idea what to expect what are your good and bad sites so you decide to fully focus on one format. Why is it most profitable for most players to focus on one format? Poker is a complex game, obviously there are more talented and average people in it but to be great in something you need to focus on it because of it complexity. Think of it as athletics. 100 meter sprinter and long jumper for sure wouldn't be doing to bad in each other disciplines if we compare them to a general population but it is just impossible for them to do both things as their career. Same is with poker you want to be great dedicate your time on your chosen format.
After some period of time you can get bored or just unmotivated with format you were playing for a long time so you decide to make a swtich. Obviously poker is poker and you will have some skillset you accumulated from your first format that will be extremely useful in new one but that's it now your main focus is new format and you completely focus on it.
Finally, there are schools of thoughts in poker on what is better mixing formats or sticking to just one and this post is more or less my attempt of answering it which for sure doesn't have to be the correct one but i strongly believe focus is absolutely crucial in any human activity and therefore do not focus on many different things while you already have established some knowledge and your priorities. As i said when you are newbie things are different, you want to give yourself a chance to know how many possibilites there are and most importantly give yourself time so the answer of specializing comes naturally.
Hope this helped to someone, Enes.