Combined XIs have a few distinct problems.
1. They're a collection of players from different teams and eras so would likely be a complete mess on the pitch.
2. They're fraught with fan bias.
3. With only 11 spots to fill, you're likely to leave out about 100 excellent footballers who would very easily put together a combined XI that would thrash your own XI..
4. Debate and discussion is dominated by fellas claiming players from days gone by are far better than modern players just because they say so. Usually unprovable and usually steeped in nostalgia.
Apart from that, great exercise...