The iFixit prices for the individual TR6 and TR10 (also called T6H and T10H, respectively) screw drivers are a bit steep. You can order a set of Torx security screw drivers for the price of those two.
The 2018 Mini with 8GB would be configured as 2x4GB. The memory modules don't have to be same size (so you could use a 4GB + 16GB one), as long both modules have the same speed/timings. However if you don't use a matched paired of memory modules, then memory access will be limited to single channel bandwidth. With matching memory modules the system will use dual channel transfers (basically, the system will stripe physical memory between the two physical RAM modules giving better throughput). Non-memory intensive applications won't see much of a difference performance wise if you're using unmatched RAM module sizes. However, memory intensive applications (e.g., video/photo editors, games) could take a big performance hit.
If you're really worried about it and want to make sure the speed/timings are going to match, you'll want to pull out the existing RAM modules and get the manufacturer and model information. That will give you the full specs of the current sticks and then you can buy memory with the matching specs. Apple uses JEDEC standards compliant memory, so any RAM modules with the same JEDEC markings memory should work fine.
The memory in my 2018 mini was manufactured by Micron (product: MTA4ATF51264HZ-2G6E3) with the following specs: 4GB 1RX16 PC4-2666V-SC0-11
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