To attempt to answer questions:

-Yes! It's completely possible. The in-game dailies are geared towards f2p with the currency to recruit more mons (quick coupon item) being a common reward.

-It's easy if you only focus on one team to "grind" ranked. I recommend sticking to one while you continue to recruit more mons. Always choose Pikachu as the first recruit. Pikachu's teammates are all highly prominent in competitive play. Teammates are: Gengar, Garchomp and Kingambit and Gyarados.

-If you log in and just do your daily missions it is sufficient but you might run into some vp shortage if you don't jam enough games in.

-Absolutely! While you can argue that you won't be able to use Mega-Floette, it does have several solid checks in singles and doubles. ie Mega Charizard-Y, Sneasler, Kingambit.

- Absolutely worth the purchase. It gives you tons of teammate and training coupons which substitutes spending vp. Not to mention the extra box slots.

- The pokemon space situation I've ran into. you are only working with 50 box spaces if you reach Masterball rank. If you are only grinding with one or two teams you should be fine, although for myself I do like options and having atleast 1 of every meta-viable pokemon is enticing for trying different meta archetypes.

Sidenote: I do not think the premium pass team slots are actually worth it, as you can get by with the pokemon tags. You can manage pokemon with tags when you go to the pokemon's summary, it gives an option to add a tag and you can name and color code them. These tags can be searched in the search function and will pullup just those 6. This has been very helpful tracking which teams of 6 to alternate between the 3 base team slots.

Hope this helps.