No simple answer especially as I suspect you may have some mixed information [but I accept my knowledge is only a good as the what I have read]. What follows has been summarised from a few places including the sites listed below:
In November 1799 Napoleon was in Paris leading the coup d'etat from which he became Consul. Christmas 1799 he became 1st Consul.
As for his bodyguard, there was his personal one "the Guides à cheval", [Company of mounted guides] formed in May 1796 following a raid by Austrian Hussars at [disputed depends what you read] from which he only just evaded capture.
Once he became 1st Consul he merged the Guides with the Garde du Directoire [Guard of the Directory] and others to become a single unit consisting ofinfantry and cavalry the Gardes des Consuls [Guard of the Consulates] that would later became the foundation of the Imperial Guard. Following the merger the Guides were renamed as the Escadron de Chasseurs-à-Cheval de La Garde Consulaire [Company oflight cavalrymen of the Consular Guard] then later the Chasseurs à Cheval de la Garde Imperiale [light cavalrymen of Imperial Guard], one of several cavalry units of the Imperial Guard.
Early in 1800 Napoleon started his Italian campaign and the Gardes des Consuls would be involved [infantry and cavalry] in the Battle of Marengo (14 June 1800) from which the Guard became famous and it appears it was therenamed "the Guides à cheval" company that was present during the battle and led one of the final cavalry charges that contributed so much to Napoleon's victory. Perhaps that is the battle your ancestor was involved in.
There is quite a bit to read and the sites below are worth noting for anyone with an interest in the Napoleonic period. see http://www.napoleonseries.org/index.cfmhttp://www.napoleonseries.org/index.cfmgo to Reference > Military Sources > Organization & Dress > [scroll down to France, Napoleon's Imperial Guard 1892-1915 AND further down to Napoleon's use of the Imperial Guard] Also see http://web2.airmail.net/napoleon/index.htmlhttp://web2.airmail.net/napoleon/index.html[down page after text are links to detailed information about units]
Between them you should be able to work out exactly what you seek and if he did take part in the Battle of Marengo and was wounded, you may be lucky andfind your ancestors name.
Finally, you may wish to note this site for future reference http://www.memoiredeshommes.sga.defense.gouv.fr/index_en.htmhttp://www.memoiredeshommes.sga.defense.gouv.fr/index_en.htm , the French Ministry of Defence "Memorial" site. There appears to be a FULL database of the Imperial Guard and Infantry of the Napoleonic period under construction and although the site is convertible into 4 languages one notice remains in French. My French is not that good but I think it says "Due to ongoing research they are unable to give a definite completion date for this part of the site" If you scroll down the text on the "Guard & Line Infantry 1st Empire you will gain some more insight into how the original records are stored etc.