There seems to be good evidence placing a unit of 7th lancers in both Spain and Russia at the same time.
Is it possible that there were two 7th Lancer Regiments, both of which consisted primarily of Poles?
1. 7e chevaux-legers-lanciers (ex- Vistula Legion) in Spain, a unit composed of Poles but in the pay of France as part of the "regiments etrangeres" component of the French Army (as was the Vistula legion);
2. 7e Lanciers, a Polish unit which was a component of the Grand Dutchy of Warsaw's Army and served in the IVe corps de calvalerie during teh Russian Campaign.
Perhaps someone more knowledgeable on regimental history than I could comment on this hypothesis.