Is Pandemic harder with fewer players?
Actually, I've found that it's easier with fewer players.  My husband and I play two-player games with six epidemics and win about half our games, but when we play with two other (competent) players five epidemics gives us that win rate and six is hard.

While, with fewer players, you have fewer roles in the game, that's offset by not having so many other actions between your turns (that can completely change the board and thus your plans).  Some roles work better the more turns that role gets, e.g. dumpster-diving abilities and once-per-turn special actions.  You'll run through the deck at the same rate no matter how many players there are.

The medic is not necessary, and sometimes having that role in a two-player game is counter-productive.  Remember tha the goal is curing diseases; it's easy for the medic to get so pre-occupied with the board that he doesn't contribute enough to cures.  Medic as one of four roles is great; medic as one of two requires restraint.

In our games we deal two roles to each player and let the player choose one (consultation is allowed).  That hasn't hurt our two-player games even though the medic will only be an option about a third of the time.