Multiple shipping routes

i am currently building my site and need to make sure i get this part correct and its one of the reasons i went away from my last package

i sell multiple items, and in major ways ie:

we are an aquatic company selling both fish and the equipment, i want to handle the shipping for live animals seperately if possible

i am not sure exactly how we plan to run it on this site yet but we will have issues with a standard table rate, let me try and explain

we use royal mail for snails etc and use a special service weighted up to 2kg for £6.95 and we also do food gravel etc problem i had with the last site was that small "dry goods" were fine as we simply set a standard table rate then set the livestock to be higher weights by 1000kg this worked 90% of the time but if some one was to add something with a bit more weight then it would be added to be shipped within the livestock rate, causing a few issues.

i was wondering about creating 2 seperate shipping rules, so that all livestock would be handled at one rate and dry goods handled seperately

i know you can create seperate rules but will they simply be added togther in the shopping cart or whilst one over right the other ?

any other ideas how to handle this ?