Not sure what you mean by "big unity" here.  You may be thinking of Unity Express there which is a totally different product line.

No, Unity Connection does not conect to Domino.  Unity can use either Exchange or Domino as it's back end and runs on Windows.  Unity Connection has it's own mailstore and runs on Linux.

Unity Connection has several features Unity does not (full speech rec, more robust language support, more granular schedules, better auto attendant functionality etc...), Unity does unified message (i.e. you get your email and voice mail all stored in Exchange or Domino - no seperate stores) - Both scale pretty large (Unity goes somewhat bigger than Unity Connection at this point but Connection is not "lite" or "small" here by any stretch).

Here's a link with some breakdown details on Unity vs. Connection vs. Unity Express.

http://www.cisco.com/en/US/prod/collateral/voicesw/ps6789/ps5745/ps2237/product_data_sheet0900aecd806bfc37_ps6509_Products_Data_Sheet.html

Be aware that both Unity and Unity Connection have 8.0 versions in EFT currently and one of the features in both is a pumpued up digital networking capability between the two systems.

(yes - naming all 3 products "Unity" was perhaps not wise - but I'm just an engineer, I have no fancy marketing degree so I can't judge such things)