Can I pick up a second weapon, like sabre after foil, without wrecking my main one?
Yes. Plenty of fencers train two weapons, and footwork, distance and fitness carry straight across. Expect a few weeks where habits bleed between them, then it settles. How much transfers depends on the pair. Foil and sabre share right of way, the rules for who gets the point when both fencers hit, so the tactical thinking overlaps. Foil and épée share point work and similar blade actions, but épée has no right of way, so a foilist's habit of attacking and trusting priority gets punished by counterattacks, and épée patience can look passive in foil.
Sabre is the biggest jump. The blade is driven by the fingers and wrist in a way that feels foreign to point fencers, the tempo is much faster, and the whole target above the waist, arms and mask included, counts. Learn it as a beginner, with a coach, rather than porting over foil actions. To protect your main weapon, keep it as your priority, train the second one on separate nights where you can, and do not start experimenting with it in the weeks before a big competition.
Gear: jacket, plastron, knickers and shoes are shared. Sabre adds a sabre with its own blade, a sabre lamé that covers the arms, a mask with a conductive bib, and a glove. USA Fencing requires an 800N-rated glove for sabre at every level of sanctioned competition. Start with basic, non-FIE blades unless you are breaking them constantly. If your class rotates all three weapons and you can only buy one for home practice, honestly, you do not need a blade at home at all. Footwork and conditioning will help more than solo bladework. If you want one anyway, buy the weapon you fence most.
Coaching a weapon you never competed in is possible too. Much of coaching is weapon-independent: fundamentals, footwork, breaking an action down and spotting faults. But you have to learn the new weapon properly, take lessons in it, watch a lot of it at a good level, and find an experienced mentor to check your work. Certification courses help. Coaches who skip that step and just teach foil with a sabre in hand are easy to spot.