Why not get all 10 at once? By splitting it up like that, you need the space and time to quarantine twice and your fish have to establish the pecking order twice.
Quarantine reduces the risk of stress related illness. Most fish have parasites or other nasties that they can co-exist with because their immune system keeps them in check. When stuffed in a bag and thrown around on conveyor belts, their immune system weakens. It weakens even more when thrown into a new tank with an established pecking order. This opens them up to new nasties (your existing stock), and their existing nasties can multiply freely because the fish is weakened (infecting your existing stock). Quarantine gives them a time to recover: no bullies, no bright light, high water quality, good food and meds if needed.
Mixing fish is always going to be "cross-contamination," but the goal is to minimize the risk. 8 week qts minimizes the risk a lot.