When you take off, you should not need much more than a very small movement of the elevator stick. If you are horsing it off with a big yank on the elevator, it can bite you.

Misalligned engine can make things bad. Since I assume you are taking off away from you, you should be able to see if the plane yaws left. If so, a little right rudder pressure will help. Then re-position the engine in the mount with more right thrust and see if that fixes it. You'll probably need to adjust the throttle cable when you do this.

Seaplane supply floats should be fine, and almost any trainer has plenty of vertical stab to be able to carry floats stably.

One last thing: double-check to see that the floats are parallel to each other and to the centerline of the airplane.