It makes no difference whether you choose 135 or 121 for your exam. The licence you get will be the same. The choice just lets you focus on an area that might be more pertinent to you if you already operate under those rules.
The flight test ('checkride' in US parlance) can be in any aircraft with a caveat: You will be limited to the category/class combination of the aircraft in which you're tested. To add another category/class you will have to do another ATP flight test.
Category: Fixed wing, rotor wing, lighter-than-air etc
Class (for fixed wing): Multi Engine Land, Multi Engine Sea, Single Engine Land, Single Engine Sea (MEL, MES, SEL, SES respectively)
There is no requirement for it to be in a multi crew type nor above any weight specification. I did my ATPL-MEL in a Duchess in the mid '90s, and in '04 a combined ATP-SEL/SES in an amphibian C180 with alightings & landings to meet the Practical Test Standard (PTS). The amphib. meant I 'only' did a single checkrides for two licences (SEL & SES).
Only a single ATP exam though - the one I did for the original ATP-MEL in '90s. For information I did the 121 version. Very simple overall. No sectorised fuel planning (a single FF for cruise), simple W&B and the like. The most difficult part was remembering the vis. limits in feet & statute miles related to the various types of lighting & approaches used in the US.
Buy a copy of the PTS! There's one published for every checkride. It lists exactly what must be covered in the flight + what may be included + what exemptions may be applied. For the ATP it will be 2 x precision approaches (ILS usually), 2 x non-precision such as NDB, VOR, G PS, LOC or one or two others. Any combination is allowed so no mandatory RMI/RBI based procedure. Also stalls, steep turns, landings/take-offs & engine failures (asymmetric or forced landing, as appropriate) during certain phases of flight.