The 'enmity' (bit of a strong word, most of the time) between Scots and the English is the typical thing between neighbours. I mean, the English and French cordially detest and/or despise each other, ditto French and Germans, Germans and Poles, Poles and Russians.... not to mention Americans and Canadians.
Although it is certainly true that the Scots have plenty of cause to dislike the English, certainly if you allow a long memory. In the 19th century England behaved appallingly towards Scotland over the Clearances - basically threw thousands of long-established Scots families off the land they had tended for generations so that it could be used for grouse (etc.) hunting. Claims of exploitation of Scots by English in the 20th century are perhaps a little harder to justify, given the extent to which English money has also helped Scotland.
Regarding Gaelic, you're MUCH more likely to hear it spoken in the north-west than in the south-east. Around Aberdeen the local dialect is Doric, while in the lowlands (south, basically) the language is Scots, which is as I understand actually recognised as a real language although in most respects it's an English dialect. I have a quite large Scots dictionary. Oh, and Scots varies considerably across the width of Scotland, which isn't much measured in miles.