Almost all the IP packets can be used interchangeably in Phase 1, but it's a smart move to use 'food' type packets for your mealtimes, and snack sort of packets for snacktimes. If you just use drinks, bars, or chips what sort of habits are you learning for maintenance? Who wants to get their bodies used to snack type foods and expecting those kinds of flavours when you start to use real foods in P3/4? Unless of course you have allergies or sensitivities to certain ingredients (ie: I have to be gluten free so most of the 'meal' type packets were out for me and I could only use a couple of the soups, the potato puree, and the omelettes in addition to the puddings and drinks - and that was it).
I take it you aren't at a clinic or using a coach or they should have told you this.
Of course, if you only can handle certain products you can still be successful, but you will be bored very easily especially if you have a fair bit to lose.
Liana