English didn't fully replace Scots in OTL, despite concerted efforts, particularly after the Act of Union. If Scotland is never dynastically and/or politically linked with England (by which I mean that James VI of Scotland never becomes monarch of England), the influence of English is going to be significantly decreased. In more recent times, we'll see that Scots will be influenced by English due to cross-border mass media-- but presumably still to a lesser degree than Scots was "anglicised" in OTL.
Since Scots was the language of Scotland's emerging sense of patriotism and national identity, Gaelic wouldn't be a threat, either. Indeed, in the period after the Wars of Scottish Independence, the key works of patriotic literature were all written in Scots. And even before James VI (who was himself a great proponent of Scots over Gaelic), the people of Scotland almost universally referenced Gaelic as Yrisch or Erse ("Irish"), and their own language (now called "Scots") as Scottis.
As such, I suspect that in the absence of any union between Scotland and Ireland, English will never get a meaningful foorhold except as a second language, and Gaelic will probably go completely extinct (instead of continuing an increasingly marginal existence, as in OTL). Conversely, without James becoming king in England, there will be no Plantation of Ulster-- or if there is, there won't be many settlers from Scotland. Meaning no Scots in Northern Ireland, and (possibly) an unchallenged supremacy of Gaelic on all that isle.