I took it for granted that this was taken care in the include file
it is to some extent , the correct tris setting for sda and scl is the power on default of input for both hw and software i2c
if the user decides to overwrite those settings its extremely difficult to rectify in an include file since the pins can be virtually
anywhere on any port depending on chip used

Next, how to import the 16x16 characters into the 5x7 library.
you cannot , the font is 5x7 hard coded

Can these co-exist?
not as a font
I tried to add your pawn example after the 5x7 data but obviously this does not work if one wants to print character 128...
i see no impediment to 5x7 chrs being added to the font to a byte sized quantity, the upper font boundary is not enforced
mixed font heights is not implemented
I attach my failed try.
if you want 16x16 fonts see here, its a completely different strategy
http://www.picbasic.co.uk/forum/showthread.php?t=24218
if you want to persist with this code, the chess pieces will need to be bitmaps
bitmaps in an array would be fairly straight forward, two for each piece type , one as solid the other an outline, with a blank w and blk too
the board could also be a bit map a piece could then be or-ed or exor-ed into a square depending piece colour and square colour