Not really sure what sweet pea substrate is. Are you speaking of pea gravel?
Choice of substrate and how it looks to each person is highly subjective, I would look at what you are trying to achieve in your set-up to determine how well a particular substrate material will work for you.
You do say you would like to use Tropica soil to benefit the plants. It is beneficial to plants; but, if you already have pea gravel and a low density of plantings you could use root tabs and/or possibly an all-in one and be able to keep the substrate you have. Depends.
Need more information on your set-up: what you hope to achieve, the plants you have, whether you have a low-tech or high-tech set-up and, of course, a picture would also be great.