Given your stated purpose, it would seem that the real question is not what combinations of consonants are possible in Pali, but rather, which of these combinations need to be represented by special characters in Sinhalese script. The answer is that this is required for pre-nasalized consonants (nt, nd, mb, etc.) and then a few others.
The following links should tell you all you need:http://www.omniglot.com/writing/sinhala.htmhttp://en.wikipedia.org/wiki/Sinhala_alphabet
Though if the modified font is intended only for Sinhalese and Pali and not for Sanskrit, then disregard sanskritic consonant clusters like kś and śrī.
Khandhānaṃ rāsaṭṭhaṃ, āyatanānaṃ āyatanaṭṭhaṃ,
Dhātūnaṃ suññaṭṭhaṃ, indriyānaṃ adhipatiyaṭṭhaṃ,
Saccānaṃ tathaṭṭhaṃ aviditaṃ karotītipi ‘avijjā’.
It prevents knowing the meaning of heap in the aggregates, the meaning of actuating in the sense-bases, the meaning of voidness in the elements, the meaning of predominance in the faculties, and the meaning of suchness in the truths, thus it is called ‘ignorance’.
(Visuddhimagga XVII. 43)