Ogniem i Mieczem - By Fire and Sword

Army lists are made basing on historical forces composition.
In casual game, as long as your opponent agrees, you can put anything you want.

Do the Lithuanian lists provide the option for more infantry or dragons? In a non tournament game can you substitute Pancerni for Petyhorcy and Wallachian's for Tartars? If your club allows the addition of mercenaries are mercenaries a better choice over dragoons or Polish infantry?

What do you mean by 'mercenaries' here?

The reason I am asking all these questions is that my friend and I are new to the game. We have 1 rule book which we share and he has it now. We do not have Deluge or Armies. We will be at Historicon next week and I would like to know what to buy. Your help is greatly appreciated. My friend has Ottomans if that helps any.

I mean German Mercenary Infantry, SWE_6

There is also the Polish-Lithuanian Town Militia PDF which in one variant lets you field 4 cossack style cavalry bases in addition to hordes of infantry.

Anyway, skirmish level games are supposed to represent raids, reconnaisance parties or garrisons, not scaled-down versions of field battles (for this you need to play Division Level or higher). But if you're just playing friendly games and not tournaments, you really can design any army list you want and let your opponent choose sides. Of course this works best if you provide the figures for both sides. I would say it's better to get started playing technically "illegal" armies than having to spend months before you can try out the game.

Another option I've found works is "bathtubbing", i.e. using the Division lists but scaling down by a factor of 1:2 or 1:4 (this means you only get 1 base or command point for every 2 or 4 in the Division list). This works best if you ignore the Divisional structure and field the forces as one big skirmish list.

There is also another option of casual game to play with more infanty forces, if you prefer such style (and if you play partners will agree and put some effort... or wait). Last year fan-made non-official rules for brigade level had been published. The problem is that they are still only in Polish, so you will need some translator (google translator won't do enough IMHO). Rules in Polsih are here (1st post - preparing a game, scenarios, tactics, victory conditions etc.; 2nd - rules of Army composition).

In general - this variant allows to combine 2 skirmishes, 2 regiments or 1 and 1 from above, and none of them can be chosen twice (without some exceptions). It merges simplicity of Skirmish level with deeper tactic of Divisional level (hidden and general tasks/missions).
The variant had been played and tested - for example during Divisional Tournament in Lublin, Aug. 2016 - and appeared (at least for me) really nice.
Official variant of this level is "under construction" and has been announced to be shown this year and - I only suppose - may be published next year.

The army lists are part of the attraction of the game.
They give a distinctive feel to the different nationalities and force people to use some historical tactics.

If you just go for the "shopping list" style of force selection, you would imbalance the game or end up with everyone fielding mixed forces of horse/foot/guns. If you want to field that type of force, you might as well go Napoleonic.

I regularly play Tartars, so cannot take any foot at all.
Otherwise I tend to field Cossacks, who are almost all foot (I don't actually have any mounted Cossacks painted yet)