not to be the naysayer, but I think you should consider doing exactly the opposite.
with a simple robot, the best course of action would be to make sure that you are as fast and manueverable as you can possibly be. this will aid you immensely in herding the small balls, and in general the fastest robot should win. yes, as such a lightweight, you probably will not win a shoving match, but why would you ever need to be in one in the first place? there is no reason for it in this game.
consider throwing an extra set of motors on your drive train. as you probably don’t want to have to design an entire gearbox for them, just put them in at as high a reduction as you can get them and scale the voltage down to bring them to the same RPMs. you don’t get as much power that way, but it is a lot easier and better than nothing.
you will want to keep the wheels you have, they are perfect for what you are doing. the only other thing i would be sure of is that there is at least one front of your robot that does not have exposed wheels. remember, wheels made to climb up on the platform will just as easily climb up on another robot and flip you over in a heartbeat.
in the end, it boils down to this. a heavier robot is better at pushing matches, but turns slower and therfore requires more power in order to be as manueverable. as a team not utilizing a four-motor gearbox, you probably wouldn’t be able to win a mushing match even if you max out your weight allotment. therefore, the other choice, and the more realistic one from what you’ve said about your bot, is to make a robot that is as light as possible. it will not win a pushing match, but it will easily be able to simply drive right around anyone trying to block you - speed alone may very well win you the game.
what ever you do, just be sure not to bite off more than you can swallow. if you can make a simple robot that is fast and manueverable, you will still be better off than at least 1/2 the teams out there. the only component that i would suggest adding on to is your plow - we’ve experimented with them, and found that in their most basic form they are extremely innefective. you could perhaps try to make a modification to the plow that corrals the balls inside it, so that when you turn/stop/reverse or do anything other than go in a straight line, the balls come with you? its up to you.